IGBTs employing a weak collector are well known. Such devices, using a non-punch through technology, and using ultra-thin float zone wafers rather than more expensive wafers with an epitaxially formed silicon layer for device junctions and buffer zones for example, are described in a paper 0-7803-3106-0/96; 1996 I.E.E.E., entitled NPT-IGBT-Optimizing for Manufacturability, in the names of Darryl Burns et al.
As described in that paper, high voltage non-punch through IGBTs (NPT-IGBTs) offer reasonable on state voltages, high short-circuit ruggedness, and minimal turn-off losses without heavy metal or E-beam lifetime killing. In addition, they have reduced cost as compared to the more conventional epitaxial IGBT because they are fabricated on low-cost bulk (float zone) silicon substrates and do not use thick, expensive epitaxial layers. The final thickness of the float zone wafers for non-punch thru IGBTs ranges from about 80 microns for 600 volt devices to 250 microns for 1700 volt devices. Even thinner wafers are needed for even lower breakdown voltages. Such wafers are fragile and subject to breakage during processing. Typically, the wafer will be about 80 microns thick for a 600 volt breakdown and 185 microns thick for a 1200 volt breakdown.
The known NPT-IGBT uses a simple, shallow low concentration backside P type implant (a “weak” or “transparent” collector) to form an emitter with low efficiency, thereby providing fast turn-off time. A collector contact including a first aluminum layer is then sintered into the bottom of the silicon wafer. In contrast, the conventional epitaxial IGBT uses an N+ epitaxial buffer layer and lifetime killing to obtain fast turn-off time.
The weak collector in an IGBT has been found to improve the relation between forward voltage drop (Vce) and switching speed, as compared to that obtained by E beam radiation or heavy metal implants. Thus, in FIG. 6, the forward voltage drop Vce is shown as a function of switching energy E (which is inversely proportional to switching speed) at 150° C. The use of heavy metals such as platinum to reduce lifetime produces a reduction in Vce in FIG. 6 at the expense of switching speed. E-beam radiation provides further improved Vce characteristics. The curve in FIG. 6 shows the best available improvement in speed using present implant techniques. The use of a weak collector in an ultra thin NPT float zone material provides even better results than E-beam radiation. However, it has not been possible in the past to increase speed (reduce switching energy) into the dotted line region of FIG. 6. This is because it is difficult to control low implant doses in the range of 1E10 atoms/cm2 to 5E11 atoms/cm2. It would be very desirable to provide a device with higher switching speed for numerous applications.